Update to orchard_auth_digest computation #1742

Job	Run time
Book tests	13s
Bitrot check	2m 57s
Latest build on ubuntu-latest	1m 11s
Test on ubuntu-latest	33m 56s
Build target wasm32-wasi	2m 7s
Latest build on windows-latest	2m 29s
Test on windows-latest	35m 6s
Code coverage	19m 29s
Latest build on macOS-latest	1m 0s
Test on macOS-latest	27m 37s
Intra-doc links	3m 5s
Rustfmt	12s
	2h 9m 22s

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